Microencapsulation was first introduced by Green and Schleicher in the 1950s to produce pressure-sensitive dye microcapsules for carbonless copying paper. Since the mid-1970s, microencapsulation has become increasingly popular in pharmaceutical and food technology. Nowadays, mechanically stable capsules are used as fillers to reduce weight and impact force of mechanical parts. Buildings are passively conditioned with mortars containing encapsulated phase change materials. Handling pesticides is much safer if the hazardous substances are encapsulated. In nutrition industry, high added value additives like flavors, vitamins or fatty acids are encapsulated and screened from premature degradation. A lot of effort is invested in adopting microencapsulation technology to deliver active agents in medicine where encapsulated drugs should safely be delivered at specific target sites.
An ideal encapsulation system features an efficient loading mechanism and yields in microcapsules with tunable size and size distribution, permeability and release kinetics as well as a high specific cargo volume. Additionally, the system allows adjusting the mechanical properties of the capsules independently from their permeability.
Particle-stabilized microcapsules are a relatively new class of capsules with great potential to meet many of those specifications. Their formation is based on the emulsion templating concept first proposed by Velev et al. in 1996 [Velev, O. D., K. Furusawa, and K. Nagayama, Assembly of latex particles by using emulsion droplets as templates .1. Microstructured hollow spheres. Langmuir, 1996. 12(10): p. 2374-2384]. Thereby, capsules with defined pore sizes can be synthesized by the self assembly of particles at the interface of emulsion droplets.
Inorganic capsules belong to a special class of microcapsules. WO 2007/068127 A1 (ETH Zurich) discloses e.g. a method for the modification of particle wettability and the formation of particle-stabilized foams. This method can be applied as well for the formation of inorganic microcapsules. Compared to state of the art co-polymer, polyelectrolyte or amphiphile vesicles, inorganic microcapsules generally exhibit higher chemical and thermal resistance as well as improved environmental compatibility. Their release kinetics can be tailored by using coarser or finer particles in order to adapt the pore size within the capsule shell [Dinsmore, A. D., et al., Colloidosomes: Selectively permeable capsules composed of colloidal particles. Science, 2002. 298(5595): p. 1006-1009]. However, the finer the particles are, the weaker the capsule shells generally become.
Unfortunately, the mechanical stability of porous inorganic microcapsule shells is in general rather low what often leads to disintegration or folding of the structures upon drying. Such coupling between system properties is undesirable and leaves room for improvement.